1. Field of the Invention
The present invention relates to a scanning probe microscope in which a force exerted between a cantilever provided on its top end with a minute probe and a sample is used for measuring an uneven shape and/or a physical characteristic of a surface of the sample and, particularly, to a scanning probe microscope in which contact between a sample and a probe can be prevented.
2. Background Art
A scanning probe microscope comprises a cantilever having a minute probe on a top end thereof, displacement detecting means for detecting displacement of the cantilever, Z-axis controlling amount calculating means for calculating a controlling amount for keeping constant a displacement amount of the cantilever or an oscillation amount in oscillating the cantilever, Z-axis driving means for driving in a Z direction the cantilever or a sample on the basis of the controlling amount from the Z-axis controlling amount calculating means, XY scanning means for relatively moving the probe in a direction of an XY plane with respect to the sample, and storing means for storing the controlling amount outputted from the Z-axis controlling amount calculating means and a signal outputted from the displacement detecting means. In the scanning probe microscope, the probe scans a surface of the sample by means of the XY scanning means so as to measure an uneven shape and/or a physical characteristic of the surface of the sample. An atomic force microscope (AFM) for detecting atomic force such as a Van der Waals force exerted between a top end of a probe and a surface of a sample is known as an example of the above.
The atomic force microscope includes Z-axis controlling means for controlling a distance between a probe and a sample so that displacement of the cantilever would be kept constant. The atomic force microscope is a scanning probe microscope in which the Z-axis controlling amount is converted into an image so as to convert an uneven shape of a surface of the sample into a three-dimensional image.
When an end of a small piece sample such as a semiconductor is measured in a scanning probe microscope, in the case that there is an area in which there is no sample, the Z-axis driving means is lowered in the area with no sample to a lower limit position in a driving range for the purpose of detecting a nonexistent sample, so that the probe is vainly moved in the air for scanning. Scanning under such a condition is likely to cause breakage of the probe due to contact between the probe and the sample when a scanning speed is faster than a moving speed of the probe to a top surface of the sample after the probe detects the sample although the Z-axis driving means is retracted in order to raise the probe to the top surface of the sample when the probe detects a side surface of the sample.
Therefore, in order to avoid the contact between the probe and the sample, there is used a method for scanning at a speed low enough for the probe to safely move to the top surface of the sample or a method for observing a height of the probe in scanning so as to stop scanning once to move the probe to a safe position before restarting scanning in the case of exceeding a previously set value of limit in height of the probe (e.g., refer to JP-A-2003-166927, Paragraphs 0025-0029, FIG. 4).
The above-mentioned methods, however, have the following problems:
1) It takes a long time to conduct the measurement, thereby lowering the throughput; and
2) Scanning is stopped once in the case of using a piezoelectric element or the like as a scanning device, so that a nonlinear characteristic of the piezoelectric element causes distortion of an image.